1. Field of the Invention
The present invention relates generally to photonic modulators.
2. Description of the Related Art
Photonics involves the control of photons; it is concerned with the properties and applications of photons, especially as a medium for transmitting information. Integrated optics is the technology of integrating various optical components onto a substrate, typically with the components connected by optical waveguides. An important integrated-optic photonic component is the intensity modulator in which an applied electrical signal amplitude modulates a light carrier. The intensity modulator is often employed in photonic communication links. It is particularly suited for use in analog signal transmission applications, e.g., cable television and remote antenna installations.
The Mach-Zehnder interferometer modulator and the directional-coupler modulator are well known intensity modulators (e.g., see Bahaa Saleh and Malvin Teich, Fundamentals of Photonics, New York, John Wiley & Sons, Inc., 1991, pp. 700-709). Both modulators are based on the electro-optic effect which is the change of a material's refractive index resulting from the application of an electric field. In some materials, the refractive index changes in proportion to the applied electric field (the Pockels effect). In others, the refractive index changes in proportion to the square of the applied electric field (the Kerr effect). The Mach-Zehnder interferometer and the directional-coupler modulator are typically fabricated with materials, e.g., lithium niobate, that exhibit the Pockels effect.
In a Mach-Zehnder modulator fabricated of lithium niobate, an electric signal is placed across a first strip of lithium niobate to cause a corresponding change in the strip's refractive index. This changes the effective optical path length, and a light signal passed through this strip emerges with the light phase modulated by the electric signal. The input and output of this first strip are joined with the input and output of a parallel second unmodulated strip. As a consequence, light at the combined output is amplitude modulated by interference between a phase-modulated signal and an unmodulated signal. In construction, the strips are typically indiffused with titanium, which raises their refractive index above that of a surrounding substrate. The difference in refractive index guides the light through the strips by total internal reflection, i.e., the strips are optical waveguides.
In the directional coupler modulator, the electro-optic effect is used to control the coupling between two parallel optical waveguides. When light travels down an optical waveguide, there is some short-range lateral penetration of the light wave beyond the waveguide boundary. This lateral wave normally decays, and is thus called the "evanescent wave". Although the evanescant wave amplitude decreases rapidly beyond the boundary, a second waveguide introduced into this region will couple into the evanescent wave and provide a path for carrying away energy that otherwise would have returned to the first waveguide. The coupled waveguides thus form a directional coupler.
If the waveguides are identical and in close proximity for a sufficient coupling length, all of the energy will transfer to the second waveguide. Two parameters govern the strength of this coupling process: the coupling coefficient c (which depends on the dimensions, operating wavelength and refractive indices) and the mismatch in propagation constants. The latter parameter is controlled by the difference in refractive indices. Applying an electric signal across the waveguides, in the coupling region, increases the refractive index in one and decreases the refractive index in the other. The exchange of energy between the two waveguides can be controlled across a range bounded by the point where all energy is transferred and the point where no energy is transferred.
The light-transmittance transfer function of the Mach-Zehnder modulator has an inflection point defined as a point of maximum slope and zero curvature. If the modulator is operated at this inflection point, its best linearity and sensitivity are obtained, its production of second harmonics is reduced to zero, and its third-order intermodulation products are minimized. For the directional coupler modulator, the choice of an operating point is not as simple. There is a first operating point for best linearity and sensitivity, another for the least production of second harmonics, and a third for best third-order intermodulation performance.
Typically, a bias voltage is applied to place an intensity modulator at a selected operating point. Because of device instabilities, e.g., caused by temperature and mechanical stress, the voltage required to establish this operating point changes over time (primarily due to changed effective path lengths in the Mach-Zehnder modulator and changed coupling coefficient in the directional coupler modulator). To counter this drift, the bias voltage must be constantly adjusted. In experimental setups this may be done manually. For stand-alone RF links that operate continuously, an automatic bias control is needed.
A prior art bias control method injects an electrical pilot tone into the modulator and detects the second harmonic of this signal at the modulator output (M. G. Lee, et al., "New Robust Bias Control Method for Optical Modulators", SPIE Symposium Digest on Optical and Digital GaAs Technologies, Vol. 1291, pp. 55-65). The bias voltage is then adjusted until the second harmonic of this pilot tone is minimized. This method is effective in bringing the modulator to an inflection point on the transmittance transfer function.
However, the circuitry is complicated: a pilot tone must be generated, its second harmonic measured, and an error signal developed from this measurement. If it is desired to reduce third-order intermodulation for a directional coupler modulator, then the control circuit would have to be modified to maintain a different operation point, e.g., by minimizing the pilot tone third harmonic. Finally, this bias control method involves a number of circuits with the consequent use of considerable power. In many modulator applications, e.g., remote antennas, a permanent power source is not available and power consumption is, therefore, a major concern.